Introduce rounding option

[Drvi]

This is useful for FixedDecimals, but it would also be useful when parsing DateTimes with nanosecond resolution, at least I always had to import them as strings, strip the last couple of digits, and parse again since which is just poor mans rounding. nothing, the default value for rounding, should mark the value as invalid if it cannot be represented exactly without rounding (do we need INEXACT code?)

Best paired with a Conf/TypeParser approach a la #131 because for some columns rounding might be acceptable but for others, it might not.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c142819) 88.49% compared to head (028e189) 88.49%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #171   +/-   ##
=======================================
  Coverage   88.49%   88.49%           
=======================================
  Files          10       10           
  Lines        1852     1852           
=======================================
  Hits         1639     1639           
  Misses        213      213           

Impacted Files	Coverage Δ
src/Parsers.jl	87.29% <100.00%> (ø)
src/floats.jl	89.03% <100.00%> (ø)

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